1. Field of the Invention
The present invention relates to the oligomerization of alkenes, e.g., the oligomerization of normal butenes to produce primarily octenes. More particularly the invention relates to the oligomerization of 1-butene and 2-butene contained in a mixed C4 stream which has been depleted of isobutenes such as from an MTBE unit or isobutene purification units. More particularly the invention relates to the oligomerization of butenes over ZSM-57 zeolite catalyst in a distillation column reactor.
2. Related Art
The mixed C4 stream from an MTBE unit or an isobutene purification unit, often called a raffinate stream, contains diluted normal butenes, especially butene-1 and butene-2. These streams have been dimerized in the past in tubular reactors containing catalysts such as supported phosphoric acid (SPA) and the zeolites ZSM-22 and ZSM-57. However the reaction conditions have been severe, e.g., temperatures of between 330° F. to 482° F. and pressures of between 1,000 psig to 1,215 psig.
Besides the reaction conditions the catalysts have previously had short lives. The SPA catalyst produces only about 333 tons of oligomers per ton of catalyst and has a useful lifetime of 2–3 weeks on stream, after which the catalyst must be discarded. The zeolite catalysts have longer lifetimes (3–4 months), but lose activity and must be regenerated off-line at considerable expense.
The selectivity of the above-mentioned catalysts is lower than ideal. The desired product from butenes is octenes, which can be converted to isononyl alcohols. Higher oligomers, such as C12 olefins are useful to the extent that profitable outlets, e.g., tridecyl alcohol or isopar solvents, can be found.
Typical results of the selectivity of the above catalysts in the tubular reactors are shown below in TABLE I:
TABLE ICatalystZSM-22ZSM-22ZSM-57ZSM-57SPA@ 94%*@ 50%*@ 94%*@ 50%*Selectivity,Mol %Paraffin<166<1<1C6 Olefin1<1<1<1<1C7 Olefin4<1<1<1<1C8 Olefin4550707888C9 Olefin91.5<13<1C10–C11132<11<1OlefinC12 Olefin222718107C12+ Olefin413572*Olefin conversion per pass
Finally, it should be noted that isobutenes have been oligomerized over acid cation exchange resin catalysts in distillation column reactors in combination with boiling point reactors as disclosed in U.S. Pat. Nos. 4,242,530 and 5,003,124.
The present invention provides higher conversion per pass than in other processes with higher selectivity. A further advantage is that the present process operates under less severe conditions of temperature and pressure than prior commercial oligomerization processes using ZSM-57 catalyst. Still another advantage is a much longer online time before turnaround to regenerate the catalyst and potentially longer catalyst life. It is a feature of the present invention that the catalyst can be regenerated and enhanced in situ, thus providing even greater efficiency and cost savings.